1. Field of the Invention
The invention relates in general to hydraulic systems which utilize multi-stage or telescopic jacks, and more specifically to such jacks having a cylinder, an intermediate section, and a plunger section, with the intermediate and plunger sections moving in synchronism.
2. Description of the Prior Art
Multi-stage or telescopic hydraulic jacks of the synchronized type are suitable for such hydraulic systems as hydraulic elevators. The work "synchronized" refers to the fact that the intermediate and plunger sections operate simultaneously with equal movement, as opposed to successively. To extend the jack, hydraulic oil is forced into a first chamber between the cylinder or outer section and an end of the intermediate section. The oil in a second chamber which surrounds an end of the plunger section is sealed, and is referred to as the "trapped oil space". When oil is forced into the first chamber under pressure, it causes the intermediate stage to extend. Since the oil in the trapped oil space cannot compress, the plunger stage must extend to maintain the volume of the trapped oil space constant. The jack components are dimensioned such that one inch movement of the intermediate stage causes one inch movement of the plunger stage.
First, second and third seals, through which one stage moves relative to another, define the trapped oil space. The first and second seals are disposed between the inside wall of the outer cylinder section and the outside wall of the intermediate section, and the third seal is disposed between the inside wall of the intermediate section and the outside wall of the plunger section. Since the stages slide through the seals, some oil necessarily must leak past the seals, and when the oil leakage from the trapped oil space is great enough, the intermediate and plunger sections get out of synchronism, i.e., they no longer move uniformly together at the lower end of the travel range, and the maximum extension of the plunger section is reduced. Thus, in an elevator system, for example, the elevator car may be unable to reach the uppermost floor of the associated building when the leakage reaches a predetermined volume.
Since it is not desirable to manually replenish the oil in the trapped oil space, either periodically or when needed, because of the cost as well as the out-of-service time, the prior art has proposed many different arrangements for automatically replenishing the oil in the trapped oil space. For example, an external reservoir in communication with the trapped oil space, with a check valve, has been proposed. However, this requires that the reservoir be periodically filled. The most common prior art arrangement is some form of internal valving between the first and second chambers. The valving may automatically operate to replenish the oil in the trapped oil space due to the reduced spacing between the inner end of the plunger section and the inner end of the intermediate section when oil is required, or, the need for oil may be detected externally by a switch which modifies the operation of the jack to cause it to fully retract and operate internal replenishment valving.
While these prior art arrangements operate satisfactorily, it would be desirable to provide a new and improved hydraulic system which (a) eliminates the need for external reservoirs, which require replenishment, (b) which does not require an "abnormal" operation of the jack, i.e., a modification of the normal operation of the jack, and (c) which eliminates internal valving, which is difficult and thus costly to repair and maintain.